The therapeutic effect of L-5-hydroxytryptophan (L-5HTP), the precursor of serotonin, in combination with a peripheral decarboxylase inhibitor, carbidopa, (MK 486) will be evaluated in patients with diseases associated with myoclonic muscle movements (eg. intention myoclonus, progressive, myoclonus epilepsy, essential myoclonus, etc.) and in seizure disorders. The metabolism of L-5HTP in these patients will be evaluated by measurement of CSF 5-hydroxyindoleacetic acid (5HIAA) and blood and urinary serotonin, 5HIAA and 5HTP. The dosage schedule, safety and clinical and biochemical side effects of this drug combination will be established. Investigations of the mechanism of action of L-5HTP plus carbidopa in relieving myoclonus will be initiated in animal models of myoclonus.